The present invention relates to a thixocast casting material, a process for preparing a thixocast semi-molten casting material, a thixocasting process, an Fe-based cast product, and a process for thermally treating an Fe-based cast product.
In carrying out a thixocasting process, a procedure is employed which comprises heating a casting material into a semi-molten state in which a solid phase (a substantially solid phase and this term will also be applied hereinafter) and a liquid phase coexist, filling the semi-molten casting material under a pressure into a cavity in a casting mold, and solidifying the semi-molten casting material under the pressure.
An Fexe2x80x94Cxe2x80x94Si based alloy having a eutectic crystal amount Ec set in a range of 50% by weightxe2x89xa6Ecxe2x89xa670% by weight is conventionally known as such type of casting material (see Japanese Patent Application Laid-open No.5-43978). However, if the eutectic crystal amount Ec is set in a range of Ecxe2x89xa750% by weight, an increased amount of graphite is precipitated in such alloy and hence, the mechanical properties of a cast product is substantially equivalent to those of a cast product made by a usual casting process, namely, by a melt producing process. Therefore, there is a problem that if the conventional material is used, an intrinsic purpose to enhance the mechanical properties of the cast product made by the thixocasting process cannot be achieved.
If a thixocast casting material made by utilizing a common continuous-casting process can be used, it is economically advantageous. However, a large amount of dendrite exists in the casting material made by the continuous-casting process. The dendrite phases cause a problem that the pressure of filling of the semi-molten casting material into the cavity is raised to impede the complete filling of the semi-molten casting material into the cavity. Thus, it is impossible to use such casting material in the thixocasting. Therefore, a relatively expensive casting material made by a stirred continuous-casting process is conventionally used as the casting material. However, a small amount of dendrite phases exist even in the casting material made by the stirred continuous-casting process and hence, a measure for removing the dendrite phases is essential.
In carrying out the thixocasting process, a semi-molten casting material prepared in a heating device must be transported to a pressure casting apparatus and placed in an injection sleeve of the pressure casting apparatus. To carry out the transportation of a semi-molten casting material, for example, a semi-molten Fe-based casting material, a measure is conventionally employed for forming an oxide coating layer on a surface of the material prior to the semi-melting of the Fe-based casting material, so that the oxide coating layer functions as a transporting container for the main portion of the semi-molten material (see Japanese Patent Application Laid-open No.5-44010). However, the conventional process suffers from a problem that the Fe-based casting material must be heated for a predetermined time at a high temperature in order to form the oxide coating layer and hence, a large amount of heat energy is required, resulting in a poor economy. Another problem is that even if a disadvantage may not be produced, when the oxide coating layer is pulverized during passing through a gate of the mold to remain as fine particles in the Fe-based cast product, and if the oxide coating layer is sufficiently not pulverized to remain as coalesced particles in the Fe-based casting material, the mechanical properties of the Fe-based cast product are impeded, for example, the Fe-based cast product is broken starting from the coalesced particles.
The present inventors have previously developed a technique in which the mechanical strength of an Fe-based cast product can be enhanced to the same level as of a carbon steel for a mechanical structure by finely spheroidizing carbide existing in the Fe-based cast product of an Fexe2x80x94Cxe2x80x94Si based alloy after the casting, i.e., mainly cementite, by a thermal treatment. Not only the finely spheroidized cementite phases but also graphite phases exist in the metal texture of the Fe-based cast product after the thermal treatment. The graphite phases include ones that exist before the thermal treatment, i.e., ones originally possessed by the Fe-based cast product after the casting, and ones made due to C (carbon) produced by the decomposition of a portion of the cementite phases during the thermal treatment of the Fe-based cast product. If the amount of the graphite phases exceeds a given amount, there arises a problem that the enhancement of the mechanical strength of the Fe-based cast product after the thermal treatment is hindered.
There is a conventionally known Fe-based cast product having a free-cutting property and made of a flake-formed graphite cast iron. However, the flake-formed graphite cast iron has a difficulty in that the mechanical property thereof is low, as compared with a steel. Therefore, measures for spheroidizing the graphite and increasing the hardness of a matrix have been employed to provide a mechanical strength equivalent to that of the steel. However, if such a measure is employed, there arises a problem that the cutting property of the Fe-based cast product is largely impeded. This is because the graphite phases precipitated in crystal grains is coagulated into a crystal grain boundary due to the spheroidizing treatment and hence, the graphite does not exist in the crystal grains, or even if the graphite exists, the amount thereof is extremely small, and as a result, the cutting property of a matrix surrounding the crystal grains is good, while the cutting property of the crystal grains is poor, whereby a large difference is produced in cutting property between the matrix and the crystal grains.
It is an object of the present invention to provide a thixocast casting material of the above-described type, from which a cast product having mechanical properties enhanced as compared with a cast product made by a melt casting process can be produced by setting the eutectic crystal amount at a level lower than that of a conventional material.
To achieve the above object, according to the present invention, there is provided a thixocast casting material which is formed of an Fexe2x80x94Cxe2x80x94Si based alloy in which an angled endothermic section due to the melting of a eutectic crystal exists in a latent heat distribution curve, and a eutectic crystal amount Ec is in a range of 10% by weightxe2x89xa6Ecxe2x89xa650% by weight.
A semi-molten casting material having liquid and solid phases coexisting therein is prepared by subjecting the casting material to a heating treatment. In the semi-molten casting material, the liquid phase produced by the melting of a eutectic crystal has a large latent heat. As a result, in the course of solidification of the semi-molten casting material, the liquid phase is sufficiently supplied around the solid phase in response to the solidification and shrinkage of the solid phase and is then solidified. Therefore, the generation of air voids of micron order in the cast product is prevented. In addition, the amount of graphite phases precipitated can be reduced by setting the eutectic crystal amount Ec in the above-described range. Thus, it is possible to enhance the mechanical properties of the cast product, i.e., the tensile strength, the Young""s modulus, the fatigue strength and the like.
In the casting material in which the eutectic crystal amount is in the above-described range, the casting temperature (temperature of the semi-molten casting material and this term will also be applied hereinafter) for the casting material can be lowered, thereby providing the prolongation of the life of a casting mold.
However, if the eutectic crystal amount Ec is in a range of Ecxe2x89xa610% by weight, the casting temperature for the casting material approximates a liquid phase line temperature due to the small eutectic crystal amount Ec and hence, a heat load on a device for transporting the material to the pressure casting apparatus is increased. Thus, the thixocasting cannot be performed. On the other hand, a disadvantage arisen when Ecxe2x89xa750% by weight is as described above.
The present inventors have made various studies and researches for the spheroidizing treatment of dendrite phases in a casting material produced by a common continuous-casting process and as a result, have cleared up that in a casting material in which a difference between maximum and minimum solid-solution amounts of an alloy component solubilized to a base metal component is equal to or larger than a predetermined value, the heating rate Rh of the casting material between a temperature providing the minimum solid-solution amount and a temperature providing the maximum solid-solution amount is a recursion relationship to a mean secondary dendrite arm spacing D, in the spheroidization of the dendrite phase comprised of the base metal component as a main component.
The present invention has been accomplished based on the result of the clearing-up, and it is an object of the present invention to provide a preparing process of the above-described type, wherein at a stage of heating a casting material into a semi-molten state, the dendrite phase is transformed into a spherical solid phase having a good castability, whereby the casting material used in the common continuous-casting process can be used as a thixocast casting material.
To achieve the above object, according to the present invention, there is provided a process for preparing a thixocast semi-molten casting material, comprising the steps of selecting a casting material in which a difference g-h between maximum and minimum solid-solution amounts g and h of an alloy component solubilized to a base metal component is in a range of g-hxe2x89xa73.6 atom %, said casting material having dendrite phases comprised of the base metal component as a main component; and heating the casting material into a semi-molten state with solid and liquid phases coexisting therein, wherein a heating rate Rh (xc2x0C./min) of the casting material between a temperature providing the minimum solid-solution amount b and a temperature providing the maximum solid-solution amount a is set in a range of Rhxe2x89xa763xe2x88x920.8D+0.013D2, when a mean secondary dendrite arm spacing of the dendrite phases is D (xcexcm).
The alloys with the difference g-h in the range of g-hxe2x89xa73.6 atom % include an Fexe2x80x94C based alloy, an Alxe2x80x94Mg alloy, an Mgxe2x80x94Al alloy and the like. If the casting material formed of such an alloy is heated at the heating rate Rh between both these temperatures, the diffusion of the alloy component produced between both the temperatures to each of the dendrite phases is suppressed due to the high heating rate, whereby a plurality of spherical high-melting phases having a lower density of the alloy component and a low-melting phase surrounding the spherical high-melting phases and having a higher density of the alloy component appear in each of the dendrite phases.
If the temperature of the casting material exceeds the temperature providing the maximum solid solution amount, the low-melting phase is molten to produce a liquid phase, and the spherical high-melting phases are left as they are, and transformed into spherical solid phases.
However, if g-h less than 3.6 atom %, or if Rh less than 63xe2x88x920.8D+0.013D2, the above-described spheroidizing treatment cannot be performed, whereby the dendrite phases remain. In a temperature range lower than the temperature providing the minimum solid-solution amount, the spheroidization of the dendrite phases does not occur.
It is an object of the present invention to provide a preparing process of the above-described type, wherein a semi-molten casting material, particularly, a semi-molten Fe-based casting material can be prepared within a transporting container by utilizing an induction heating, and the Fe-based casting material can be heated and semi-molten with a good efficiency by specifying a container forming material and the frequency of the induction heating, and the temperature retaining property of the semi-molten Fe-based casting material can be enhanced.
To achieve the above object, according to the present invention, there is provided a process for preparing a thixocast semi-molten casting material, comprising the steps of selecting an Fe-based casting material as thixocast casting material, placing the Fe-based casting material into a transporting container made of a non-magnetic metal material, rising the temperature of the Fe-based casting material from the normal temperature to Curie point by carrying out a primary induction heating with a frequency f1 set in a range of f1 less than 0.85 kHz, and then rising the temperature of the Fe-based casting material from the Curie point to a preparing temperature providing a semi-molten state of the Fe-based casting material with solid and liquid phases coexisting therein by carrying out a secondary induction heating with a frequency f2 set in a range of f2xe2x89xa70.85 kHz.
The semi-molten Fe-based casting material is prepared within the container and hence, can be easily and reliably transported as placed in the container. The container can be repeatedly used, leading to a good economy.
The Fe-based casting material is a ferromagnetic material at normal temperature and in a temperature range lower than the Curie point, while the container is made of a non-magnetic material. Therefore, in the primary induction heating, the temperature of the Fe-based casting material can be quickly and uniformly risen preferentially to the container by setting the frequency F1 at a relatively low value as described above.
When the temperature of the Fe-based casting material is risen to the Curie point, it is magnetically transformed from the ferromagnetic material to a paramagnetic material. Therefore, in the temperature range higher than Curie point, the temperatures of the Fe-based casting material and the container can be both risen by conducting the secondary induction heating with the frequency f2 set at a relatively high value as described above. In this case, the rising of the temperature of the container has a preference to the rising of the temperature of the Fe-based casting material. Therefore, the container can be sufficiently heated to have a temperature retaining function, and the overheating of the Fe-based casting material can be prevented, thereby preparing a semi-molten Fe-based casting material having a temperature higher than a predetermined preparing temperature, namely, a casting temperature which is a temperature at the start of the casting.
In the subsequent course of transportation of the semi-molten Fe-based casting material, the temperature of the material can be retained equal to or higher than the casting temperature by the heated container.
When the temperature T1 of the Fe-based casting material reaches a point in a range of T2xe2x88x92100xc2x0 C.xe2x89xa6T1xe2x89xa6T2xe2x88x9250xc2x0 C. in the relationship to the preparing temperature T2 in the course of rising of the temperature by the secondary induction heating, the heating system is switched over to a tertiary induction heating with a frequency f3 set in a range of f3 less than f2, to cause the preferential rising of the temperature of the Fe-based casting material. Thus, the drop of the temperature of the semi-molten Fe-based casting material during transportation thereof can be further inhibited.
If the frequency f1 in the primary induction heating is equal to or higher than 0.85 kHz, the rising of the temperature of the Fe-based casting material is slowed down. If the frequency f2 in the secondary induction heating is lower than 0.85 kHz, the rising of the temperature of the Fe-based casting material is likewise slowed down.
It is an object-of the present invention to provide an Fe-based cast product of the above-described type, wherein the amount of graphite phases produced by the thermal treatment is substantially constant and hence, the amount of graphite phases produced by a casting can be suppressed to a predetermined value, thereby realizing the enhancement in mechanical strength by the thermal treatment.
To achieve the above object, according to the present invention, there is provided an Fe-based cast product, which is produced using an Fexe2x80x94Cxe2x80x94Si based alloy which is a casting material by utilizing a thixocasting process, followed by a finely spheroidizing thermal treatment of carbide, wherein an area rate A1 of graphite phases existing in a metal texture of said cast product is set in a range of A1 less than 5%.
With the above configuration of the Fe-based cast product, in the area rate A1 of the graphite phases lower than 5% after the casting, the area rate A2 of the graphite phases after the thermal treatment can be suppressed to a value in a range of A2 less than 8%, thereby enhancing the mechanical strength, particularly, the Young""s modulus, of the Fe-based cast product to a level higher than that of, for example, a spherical graphite cast iron.
In the area rate A1 of the graphite phases after the casting equal to 0.3%, the area rate A2 of the graphite phases after the thermal treatment can be suppressed to a value equal to 1.4%, thereby enhancing the Young""s modulus of the Fe-based cast product to the same level as that of a carbon steel for a mechanical structure.
However, if the area rate Al of the graphite phases after the casting is equal to or larger than 5%, the mechanical strength of the Fe-based cast product after the thermal treatment is substantially equal to or lower than that of the spherical graphite cast iron.
It is an object of the present invention to provide a thixocasting process of the above-described type, which is capable of mass-producing an Fe-based cast product of the above-described configuration.
To achieve the above object, according to the present invention, there is provided a thixocasting process comprising a first step of filling a semi-molten casting material of an Fexe2x80x94Cxe2x80x94Si based alloy having a eutectic crystal amount Ec lower than 50% by weight into a casting mold, a second step of solidifying the casting material to provide an Fe-based cast product, a third step of cooling the Fe-based cast product, the mean solidifying rate Rs of the casting material at the second step being set in a range of Rsxe2x89xa7500xc2x0 C./min, and the mean cooling rate Rc for cooling to a temperature range on completion of the eutectoid transformation of the Fe-based cast product at the third step being set in a range of Rcxe2x89xa7900xc2x0 C./min.
The eutectic crystal amount Ec is related to the area rate of the graphite phases. Therefore, if the eutectic crystal amount Ec is set at a value lower than 50% by weight and the mean solidifying rate Rs is set at a value equal to or higher than 500xc2x0 C./min, the amount of the graphite phases crystallized in the Fe-based cast product can be suppressed to a value in a range of A1 less than 5% in terms of the area rate A1. If the mean cooling rate Rc is set in the range of Rcxe2x89xa7900xc2x0 C./min, the precipitation of the graphite phases in the Fe-based cast product can be obstructed, and the area rate A1 of the graphite phases can be maintained in the range of A1 less than 5% during the solidification.
However, if the eutectic crystal amount Ec is in a range of Ecxe2x89xa750% by weight, the area rate A1 of the graphite phases assumes a value in a range of A1xe2x89xa75%, even if the mean solidifying rate Rs and the mean cooling rate Rc are set in the range of Rsxe2x89xa7500xc2x0 C./min and in the range of Rcxe2x89xa7900xc2x0 C./min, respectively. If the mean solidifying rate Rs is in a range of Rs less than 500xc2x0 C./min, the area rate A1 of the graphite phases assumes a value in the range of A1xe2x89xa75%, even if the eutectic crystal. amount Ec is set in the range of Ec less than 50% by weight. Further, if the mean cooling rate Rc is in a range of Rc less than 900xc2x0 C./min, the area rate A1 of the graphite phases lower than 5% cannot be maintained.
It is an object of the present invention to provide an Fe-based cast product having the free-cutting property of which cutting property is enhanced by dispersing a certain amount of graphite phases even in a group of fine a-grains of a massive shape corresponding to crystal grains, namely, in a massive area formed by coagulation of the fine a-grains.
To achieve the above object, according to the present invention, there is provided an Fe-based cast product which is produced by thermally treating an Fe-based cast product made by utilizing a thixocasting process using an Fe-based casting material as a casting material, the Fe-based cast product including a matrix and a large number of massive groups of fine xcex1-grains dispersed in the matrix, the Fe-based cast product having a thermally-treated texture where a large number of graphite phases are dispersed in the matrix and in each of the groups of fine xcex1-grains, and the Fe-based cast product having a free-cutting property such that a ratio of B/A of an area rate B of the graphite phases in all the groups of fine xcex1-grains to an area rate A of the graphite phases in the entire thermally-treated texture is in a range of B/Axe2x89xa70.138.
The massive groups of fine xcex1-grains are formed by the transformation of initial crystal xcex3-grains at a eutectoid temperature Te, and the graphite phases in the groups of fine xcex1-grains are precipitated from the initial crystal xcex3-grains. Further, the groups of fine xcex1-grains includes cementite phases. If the amount of graphite phases in all such massive groups of fine xcex1-grains is specified as described above, the cutting property of the groups of fine xcex1-grains can be enhanced, and the difference in cutting property between the groups of fine xcex1-grains and the matrix can be moderated. However, if B/A less than 0.138, the cutting property of the Fe-based cast product is deteriorated.
Here, the area of the matrix is represented by V. If areas of the individual groups of fine xcex1-grains are represented by W1, W2, W3 - - - wn, respectively, a sum total W of the areas of all the groups of fine xcex1-grains is represented by W=w1+w2+W3 - - -+wn. Further, areas of the individual graphite phases in the matrix are represented by x1, x2, x3 - - - xn, respectively, a sum total of the areas of all the graphite phases in the matrix is represented by X=x1+x2+x3 - - -+xn. Yet further, if areas of all the graphite phases in the individual groups of fine xcex1-grains are represented by y1, y2, y3 - - - ynn, respectively, a sum total Y of the areas of the graphite phases in all the groups of fine xcex1-grains is represented by Y=y1+y2+y3 - - -+yn.
Therefore, the area rate A of the graphite phases in the entire thermally-treated texture is represented by A={(X+Y)/(V+W)}xc3x97100 (%). The area rate B of the graphite phases in all the groups of fine xcex1-grains is represented by B=(Y/W)xc3x97100 (%).
It is another object of the present invention to provide a thermally treating process of the above-described type, which is capable of easily mass-producing an Fe-based cast product similar to that described above.
To achieve the above object, according to the present invention, there is provided a process for thermally treating an Fe-based cast product, comprising the step of subjecting an Fe-based as-cast product made by a thixocasting process to a thermal treatment under conditions where, when a eutectoid temperature of the as-cast product is Te, the thermal treating temperature T is set in a range of Texe2x89xa6Txe2x89xa6Te+170xc2x0 C., and the thermally treating time t is set in a range of 20 minutesxe2x89xa6txe2x89xa690 minutes, thereby providing a thermally-treated product with a free-cutting property.
Since the Fe-based as-cast product is produced by the thixocasting process, it has a solidified texture resulting from quenching by a mold. If such as-cast product is subjected to a thermal treatment under the above-described conditions, an Fe-based cast product having a free-cutting property of the above-described configuration can be produced.
At least one of a meshed cementite phase and a branch-shaped cementite phase is liable to be precipitated in the solidified texture. This causes deterioration of the mechanical properties of the Fe-based cast product, particularly, the toughness. Thereupon, it is a conventional practice to completely decompose and graphitize the meshed cementite phase and the like by subjecting such Fe-based as-cast product to the thermal treatment. However, if the complete graphitization of the meshed cementite phase and the like is performed, the following problem is encountered: the Young""s modulus of the Fe-based cast product is reduced, and because the thermally treating temperature is high, it is impossible to meet the demand for energy-saving.
If the Fe-based as-cast product is subjected to the thermal treatment under the above-described conditions, the meshed cementite phases and the like can be finely divided. The Fe-based cast product having the thermally-treated texture and resulting from the fine division of the meshed cementite phases and the like has a Young""s modulus and fatigue strength which are substantially equivalent to those of a carbon steel for a mechanical structure.
However, if the thermally treating temperature T is lower than Te, the thermally-treated texture cannot be produced, and the meshed cementite phase and the like cannot be finely divided. On the other hand, if T greater than Te+170xc2x0 C., the coagulation of the graphite phases out of the groups of fine xcex1-grains into the boundary is liable to be produced, and the graphitization of the meshed cementite phases and the like advances. If the thermally treating time t is shorter than 20 minutes, a metal texture as described above cannot be produced. On the other hand, if t greater than 90 minutes, the coagulation and the graphitization advance.